The present invention relates to a method for manufacturing silk fibroin powder below 3 xcexcm in an average particle diameter from silk substances.
Silk powder is used as an additive for food, beverages, cosmetics, and paints. Conventionally known methods for manufacturing silk powder, particularly, crystalline silk fibroin powder, include:
1) a mechanical method for comminuting silk yarn into powder, and 2) a chemical method in which, for example, a silk substance is dissolved in an aqueous solution containing an acid or neutral salt or the like; thereafter salt produced by neutralization is removed to form a fibroin aqueous solution; a precipitant is added into this aqueous solution to allow silk fibroin to be precipitated; and then this precipitate is dried by separation or the aforementioned aqueous solution is dried by freezing or by spraying.
As an example of powdering according to the aforementioned mechanical method, an example for obtaining 300-mesh silk powder (approximately 30 through 50 xcexcm in an average particle diameter) for use in cosmetics (Japanese Patent Publication No. Sho 27-3650) has been formerly reported and an example of obtaining silk fibroin superfine powder 3.25 xcexcm in an average particle diameter for use in paints (Japanese Laid-Open Patent Publication No. Hei 6-339924) has been reported.
However, the chemical method produces powder of a low crystalline level and thus is not available for producing crystalline silk fibroin powder having a property intrinsically provided in silk.
Silk substances have a property in that moisture absorbency and water absorbency increase as crystallinity becomes lower, so that silk absorbs moisture or water to become softened and easily transformed.
If silk powder having such low crystallinity is dried after absorption of moisture or water, the powder tightly coagulates to form very hard lumps and thus provides no value for use. Therefore, in order to hold the property intrinsically provided in silk, the powder is required to be a crystalline powder in which the structure intrinsically provided in silk yarn remains. Therefore, it can be said that the mechanical comminuting method is desirably employed as an industrial manufacturing method of crystalline powder. On the other hand, as described above, the prior art is limited to obtaining powder approximately 3 xcexcm in an average particle diameter even by the mechanical comminuting method.
That is, various types of mechanical pulverizers provide a limited size of particles by comminuting depending on the property of a raw material to be pulverized so long as the same type of pulverizers are used, whereby powder with a limitlessly small particle diameter cannot be obtained.
In general, pulverizers employing a higher impact speed provide powder having a smaller diameter, however, smaller powder diameters provide lower probability of collision of particles. This provides lower probability for particles to be applied with stress beyond the fracture speed, so that the energy efficiency of the pulverizer decreases acceleratedly with the diameter of particles equal to or less than a specific value.
Therefore, approximately 3 xcexcm is thought to be the limit in an average particle diameter provided by means of the prior art method for obtaining fine powder by comminuting silk yarn.
Concerning the application of silk substances, the present inventors formerly made it clear that silk substances match skin whereby the powder thereof is useful as an epidermal cell growth activating raw material (Japanese Patent Application No. Hei 9-333560). In addition, the fine powder of silk substances has also been expected to be useful as a raw material filler, a coating material, a raw material for cosmetics, etc. The present inventors formerly developed a method for obtaining crystalline silk fibroin powder 3 through 6 xcexcm in an average particle diameter by deteriorating a silk substance in strength through the exposure thereof to an aqueous solution of an alkali meal compound under atmospheric pressure at temperatures of 95xc2x0 C. or higher, and thereafter by performing dealkalization and drying of the resultant silk substance, and then by comminuting (U.S. Pat. No.2,615,440). Thereafter, intensive research lead us to a result in that crystalline silk fibroin powder, the powder being a superfine powder below 3 xcexcm in an average particle diameter with the structure intrinsically provided in silk, exerts a property unique thereto so as to be useful for various applications.
That is, such findings have been obtained in that the crystalline silk fibroin powder which is a superfine powder below 3 xcexcm in an average particle diameter provides excellent formability, improved adhesive property to the skin or the like, improved extensibility, and an improved sense of touch and the like, and is outstandingly suitable for applications to raw materials for use in cosmetics such as lip sticks, eyebrow paints, hair dyes, eyeliners, powder blushes, and foundations, and for applications to ink additives, resin composite raw materials, and raw materials for paints.
The present invention has been developed in view of the technical background mentioned above. Its object is to provide a method for manufacturing industrially, by mechanically comminuting silk yarn, crystalline silk fibroin powder below 3 xcexcm in an average particle diameter, which can be used for each of the aforementioned applications.
In order to achieve the aforementioned object, the present invention employs technical items as shown below. That is, the present invention lies in (1) a method for manufacturing crystalline silk fibroin powder, in which the structure of silk yarn remains, below 3 xcexcm in an average particle diameter, which is obtained by bringing a silk substance such as cocoon filaments, silk yarn, and raw silk into contact with an alkali aqueous solution under a pressure above 1 atmospheric pressure to reduce the strength of the silk substance, then by dealkalizing and drying the silk substance after that, and by comminuting the resultant silk substance thereafter.
Furthermore, the present invention lies in (2) a method for manufacturing crystalline silk fibroin powder below 3 xcexcm in an average particle diameter by bringing a silk substance such as cocoon filaments, silk yarn, and raw silk into contact with an alkali aqueous solution under a pressure of 1 through 5 atmospheric pressure at a temperature of 100xc2x0 C. through 150xc2x0 C. to reduce the tensile strength of the silk substance to around 0.02 g/d or less, then dealkalizing and drying the resultant silk substance after that, and by comminuting the dried resultant silk substance thereafter.
Still furthermore, the present invention lies in (3) the method for manufacturing crystalline silk fibroin powder below 3 xcexcm in an average particle diameter according to the aforementioned (2) in which alkalinity of said alkali aqueous solution is pH 9 through pH 12.5.
Still furthermore, the present invention lies in (4) the method for manufacturing crystalline silk fibroin powder below 3 xcexcm in an average particle diameter according to the aforementioned (2) in which impact and frictional comminution are combined in order to comminute said dried silk substance.
Still furthermore, the present invention lies in (5) the method for manufacturing crystalline silk fibroin powder below 3 xcexcm in an average particle diameter according to the aforementioned (2) in which impact and frictional comminution are combined in order to comminute said dried silk substance whereby particle sizes are classified.
Still furthermore, the present invention lies in (6) the method for manufacturing crystalline silk fibroin powder below 3 xcexcm in an average particle diameter according to the aforementioned (4) in which in order to perform comminution by combining impact and frictional comminutions; impact comminutionxe2x80x94frictional comminution, frictional-comminutionxe2x80x94impact comminution, or frictional comminutionxe2x80x94impact comminutionxe2x80x94frictional comminution is performed first in sequence to produce silk powder 4 through 15 xcexcm in an average particle diameter, and then impact comminution is performed to produce superfine powder below 3 xcexcm in an average particle diameter.
The present invention may be employed, so long as it follows the objects described in the specification, in a configuration with a combination of two or more selected from the group consisting of the aforementioned 1, 3, 4, 5, and 6, or in a configuration with a combination of two or more selected from the group consisting of the aforementioned 2, 3, 4, 5, and 6.
According to the present invention, in order to obtain crystalline silk fibroin powder below 3 xcexcm in an average particle diameter, the following configuration is employed. That is,
1) a raw material of a silk substance such as cocoon filaments, silk yarn, and raw silk
2) an alkali aqueous solution is brought into contact
3) a temperature of 100xc2x0 C. through 150xc2x0 C.
4) under a pressure of 1 through 5 atmospheric pressure
5) a configuration that reduces the tensile strength of the silk substance to around 0.02 g/d or less,
6) thereafter, the resultant silk substance is dealkalized and dried, and
7) then, the dried resultant silk substance is comminuted.
Thus, the above configuration provides crystalline silk fibroin powder below 3 xcexcm in an average particle diameter. In the present invention, it is important to treat the silk substance so that the tensile strength thereof is reduced to around 0.02 g/d or less. It is difficult to obtain powder below 3 xcexcm in an average particle diameter with a tensile strength above 0.02 g/d.
Moreover, it is important to treat the silk substance at a temperature of 100xc2x0 C. through 150xc2x0 C. and under a pressure so as to provide uniform degradation in strength.
Moreover, it is desirable to employ, in the comminuting process, a multi-stage comminuting method with two stages or more in which the impact comminution and frictional comminution are combined together.
The silk substance for use in the present invention includes cocoon filaments, raw silk, silk yarn (yarn from which sericin is excluded by degumming), and leftover lint thereof.
Moreover, the silk substance can be applied to woven fabrics, knit fabrics, non-woven fabrics, net yarn, and the like, which are formed of these respective yarns.
In order to implement the present invention, the silk substance is first treated by being brought into contact with an alkali aqueous solution at a temperature above 100xc2x0 C. preferably, at a temperature within a range of 120xc2x0 C.xc2x110xc2x0 C. under a pressure.
As an alkaline substance in the alkaline aqueous solution, sodium carbonate, sodium hydrogen-carbonate, potassium carbonate, sodium hydroxide, potassium hydroxide may be used individually or mixed for use.
The degree of the alkaline aqueous solution (alkalinity) is a pH of 9 through 12.5, preferably, a pH of 10.5 through 12.0.
With a pH less than 9, the strength cannot be reduced uniformly with efficiency, while with a pH above 12.5, an excessive dissolved portion of the silk yarn or the like reduces the rate of collection.
Sodium carbonate, an alkaline substance, has a buffering action as an aqueous solution and is very useful since an increase in concentration hardly causes it to increase the pH thereof to more than around 12.5.
In addition, a slight mixture amount of sodium hydroxide into sodium carbonate may cause the total amount of the alkaline substance used to decrease.
The silk substance in the alkali aqueous solution is treated by soaking the silk substance in the alkaline aqueous solution of a temperature above 100xc2x0 C.
The time for the silk substance to be in contact with the alkaline aqueous solution (Alkali treatment time) is the time necessary to allow the strength of the silk substance to decrease to a degree suitable for forming superfine powder regardless of whether Bombyx mori silk or wild silks. (Philosamia cynthia ricini, Antheraea yamamai, Antheraea pernyi, Antheraea mylitta and so on.)
For example, a silk substance is kept in contact with the alkaline aqueous solution until the tensile strength thereof becomes around 0. 02 g/d or less and more preferably, up to such an extent that the strength of the silk yarn is substantially immeasurable in the tensile test (0.01 g/d or less), that is, until the silk yarn loses the form thereof available for measurement.
In general, the treatment time is 0.5 through 5 hours. In cases where cocoon filaments, raw silk yarn, or silk yarn has yarn fabrics large in diameter or has sericin adhesion found therewith or in cases where the strength cannot be readily decreased such as in the case of wild silk yarns, two or three more hours need to be added or the concentration of the alkaline substance needs to be increased.
A specific time required for alkali treatment can be determined by examining the relationship between the easiness of comminution of the silk substance whose strength has been reduced through the alkali treatment, the time for the contact with the alkali, and the temperature.
What is important in reducing the strength of the silk yarn through the alkali treatment is to allow the strength of the silk yarn to decrease uniformly.
In this case, what is particularly essential is that if the silk substance to be subjected to the alkali treatment is well separated down to cocoon filaments or silk yarn, powder below 3 xcexcm in an average particle diameter may be obtained even with boiling (at the atmospheric temperature) at an alkali treatment temperature of around 100xc2x0 C., however, this will result in a variation in strength and lead to a prolonged time in alkali treatment, requiring a greater amount of alkaline substance and thus providing a significantly reduced ratio of collection.
Therefore, it is important to reduce variations in strength after the alkali treatment which depend on the portion of the silk substance such as cocoon filaments and silk yarn.
The present invention overcomes such a problem by performing the alkali treatment under a pressure above the atmospheric pressure.
A pressure above the atmospheric pressure can be obtained by putting the silk substance, an alkali substance, and water into a sealed container and then by increasing the temperature thereof to a temperature greater than the boiling temperature. The pressure ranges substantially from 1 through 5 atmospheric pressure. A pressure ranging from 1 through 3 atmospheric pressure may be preferably employed in practice particularly in industrial production.
For example, the alkali treatment for the silk substance would be carried out more uniformly under a pressure of 0.01 atmospheric pressure added to the atmospheric pressure than under the atmospheric pressure and provide a finer average particle diameter. However, it is far more effective to preferably carry out the alkali treatment under more than 0.1 atmospheric pressure added to the atmospheric pressure and, more preferably, under around two atmosphere pressure, at a temperature of 120xc2x0 C.xc2x110xc2x0 C.
The silk substance after having been subjected to the alkali treatment is separated from the alkaline aqueous solution, thereafter alkaline substances adhered thereto are removed by washing in water to be dealkalized, and then the silk substance is dried.
Neutralization with acids such as hydrochloric acid or tartaric acid can be employed for dealkalization, and are effective for dealkalization and appropriate for skin.
Moreover, addition of natural acids (such as a liquid squeezed out of a citrus, for example, lemon) after neutralization and washing in water allows the silk substance to effectively fit to skin and thus makes the substance more suitable for a powder raw material for use in cosmetics.
Drying is preferably carried out using a dryer for positive drying, however, natural drying should also be possible.
The superfine powdering of the silk substance is carried out by means of combining impact comminution (coarse comminution and superfine comminution) with frictional comminution.
For example, any of the multi-stage comminuting methods in a) through c) below are employed.
a) impact comminution (coarse comminution)xe2x86x92frictional comminutionxe2x86x92impact comminution (superfine comminution)xe2x86x92(particle size classification)
b) frictional comminutionxe2x86x92impact comminution (coarse comminution)xe2x86x92impact comminution (superfine comminution)xe2x86x92(particle size classification)
c) frictional comminutionxe2x86x92impact comminution (coarse comminution)xe2x86x92frictional comminutionxe2x86x92impact comminution (superfine comminution)xe2x86x92(particle size classification)
Particles are preferably comminuted into 4 through 15 xcexcm in an average particle diameter before the last impact comminution (superfine comminution) is carried out.
Within this range of particle diameter, the impact comminution (superfine comminution) is carried out with great efficiency.
At a result, crystalline silk superfine comminuted particles below 3 xcexcm in an average particle diameter are obtained by the superfine comminution.
In particular, powder below approximately 1 xcexcm or less can be obtained by the classification of particle sizes of the resultant silk superfine comminuted particles.
The resultant silk superfine comminuted particles provide an excellent sense of touch and are ready for forming.
In particular, powder having a diameter of around 1 xcexcm provides significantly-improved adhesion to skin and extensibility and serves as a skin protective material including raw materials for cosmetics and resin composites. In cases where the material silk substance is brought into contact with the alkali aqueous solution to be treated under a pressure at a temperature of 100xc2x0 C. through 150xc2x0 C., the treatment is carried out by soaking the silk substance into an alkaline aqueous solution in a glass container or in a pressure-resistant metallic container made of stainless steel.
At this time, in order to reduce the strength of the material silk substance uniformly, fiber-shaped silk substances such as cocoon filaments, raw silk, or silk yarn are preferably separated as much as possible.
Dealkalization after the alkali treatment is carried out by means of repeating washing and dehydrating for neutralization or by the neutralization with acids such as hydrochloric acid or tartaric acid, and thereafter washing is repeated through washing in water and a dehydration process.
In the dehydration process, a cloth-shaped filter, for example, a filter with mesh so fine as to collect fine silk particles around 0.5 xcexcm in a particle diameter is used.
In the alkali treatment process, non-crystalline portions of the material silk substance dissolve gradually into the alkali aqueous solution and the dissolved silk substance is removed by water at the time of dehydration. Accordingly, the silk substance to be comminuted is comprised of a crystalline silk substance in which the structure provided intrinsically for silk yarn remains (a xcex2-type with fibroin molecules uniaxially oriented).
In the alkali treatment under a pressure according to the present invention, the amount of an alkaline substance required is much less than that required in the case of the aforementioned U.S. Pat. No. 2,615,440 in which the alkali treatment is carried out under a normal pressure. For example, the ratio of silk to carbonic acid soda must be 1:1 in the case of U.S. Pat. No. 2,615,440, while the ratio of silk to carbonic acid soda is only 1:0.5 in the case of the present invention in which the alkali treatment is carried out under a pressure at a temperature of 120xc2x0 C.
This means that an improved effect is provided, in industrial production of crystalline superfine silk powder, whereby the number of times of washing in water and dehydration after the alkali treatment and the amount of acid required can be naturally reduced.
According to the present invention, silk fibroin superfine powder below 3 xcexcm in an average particle diameter can be manufactured from a silk substance inexpensively with efficiency.
The silk fibroin superfine powder obtained by the present invention is crystalline and has a crystalline form of a xcex2-type with fibroin molecules uniaxially oriented in the same manner as the silk yarn as described above. The powder is insoluble in water and has advantageous properties equivalent to those of silk yarn in hygroscopic property, desorptive property, moisture permeability, etc.
In particular, the powder is readily formable because of fine particles thereof, providing an improved adhesion and extensibility to the skin, and providing an extremely improved sense of touch, etc. The powder is therefore very useful for applications of raw materials for cosmetics such as lip sticks, eyebrow paints, hair dyes, eyeliners, powder blushes, and foundations, and for applications to ink additives, resin composite raw materials, and raw materials for paints.